In order to better understand the nature of underground formations surrounding a borehole, it is known to make measurements from within the borehole using tools suspended from the surface. One particular type of tool comprises a tool body or sonde having one or more extendable arms carrying sensor pads. In use, the arm or arms are extended until the pad is placed against the wall of the borehole, at which point measurements are made using the sensors on the pad. The tool can then be moved along the borehole (usually from bottom to top), dragging the pad across the borehole wall and allowing multiple measurements to be made along the length of the borehole.
One example of a tool of the type described above is the FMI tool of Schlumberger which comprises a sonde with four arms arranged in a radial array, each arm carrying a sensor pad with an array of button electrodes for making micro-resistivity measurements using the current injection principle. Details of the use and operation of such a tool can be found in U.S. Pat. No. 4,567,759. U.S. Pat. No. 6,191,588 describes a similar tool (OBMI tool of Schlumberger) for making measurements in boreholes containing non-conductive fluids using the 4-terminal principle. U.S. Pat. No. 7,066,282 describes other tools for making measurements in boreholes containing non-conductive fluids using the high-frequency current injection principle. Other forms of pad measurement are also known, such as micro-sonic and nuclear measurements for density determination.
One problem with pad tools of the type described above is that several different tools are needed to cover all eventualities of tool geometry and measurement technology. In some cases, these may only be used occasionally. It is an object of the invention to provide a tool structure that allows the re-use of common tool components as much as possible. It has been proposed previously to adopt a modular approach to borehole tool design. Examples can be found in GB2424665 and US2004/0251048.
This invention is based on the understanding that providing particular functionality only in the sensor pads and an electronics cartridge allows much of the tool structure to be common to a number of measurement types of tool configurations.